1. Field of the Invention
The present invention relates generally to an apparatus for reproducing information signals recorded in spiral or coaxial tracks of a record disc and is directed more particularly to a digital audio disc player for reproducing digital audio information recorded on a record disc.
2. Description of the Prior Art
A device for reproducing or playing back digital audio information recorded on a digital audio disc (hereinbelow simply referred to as DAD), which is also referred to as a compact disc, will be now described with reference to FIG. 1. In FIG. 1, reference numeral 1 designates a DAD which is rotated by a spindle motor 2 in the counter-clockwise direction viewed from the underside of the DAD 1. A pick-up apparatus or device 3 is located to oppose the DAD 1 from its underside and is moved usually in a radial direction in respect to the axis of rotation of the record disc or DAD 1 by a moving or transporting mechanism consisting of a worm gear 4 and a motor 5 for rotating the former. A detecting device 6 is provided for detecting the speed of rotation of spindle motor 2. The signal detected by detecting device 6 is applied to a control circuit 7, which controls motor 5, whereby motor 5 is rotated a predetermined amount at every one revolution of the DAD 1. Further, a signal indicating the position of pick-up device 3 is supplied to a control circuit 8, which controls spindle motor 2, such that the DAD 1 is rotated at a constant linear velocity.
FIG. 2 is a mainly cross-sectional linear diagram schematically showing the construction of the pick-up device 3. In the embodiment shown in FIG. 2, an outer casing 10 houses a laser light source 11 made of, for example, a semiconductor laser. The laser light beam emitted from laser light source 11 is conducted through a beam splitter 12 to an objective lens 13. The light beam passed through the lens 13 is irradiated and focused as a light beam spot on the information surface of the DAD 1. The light beam reflected from the surface of the DAD 1 passes through objective lens 13 to beam splitter 12, is reflected thereon and then supplied to a photo detector 14. The information signal carried by the reflected light beam is detected by photo detector 14 and supplied to an output terminal 15. In addition, within a portion of photo detector 14 are generated a focusing servo control signal and a tracking servo control signal, each of which will be described later.
Reference number 16 in FIG. 2 represents a hollow cylinder having upper and lower ends which are coupled to outer casing 10 of pick-up device 3 by springs 17a and 17b, respectively, thereby allowing hollow cylinder 16 to be axially movable.
Further, hollow cylinder 16 is provided with a flange 18 at, for example, a lower portion thereof. Flange 18 includes a cylindrical bobbin 19 formed coaxially with hollow cylinder 16.
Reference numeral 21 in FIG. 2 designates a magnet attached to the casing 10. Each pole of magnet 21 is respectively connected to an outer end of yokes 22a and 22b, each of which is made of magnetic material. The inner ends of yokes 22a and 22b are extended to be located near the outer and inner peripheries of the coil 20, respectively.
Within hollow cylinder 16 are further provided two plate or leaf springs 23a and 23b each of which extends parallel with respect to the other from the lower end portion of hollow cylinder 16 to the upper end portion thereof. Object lens 13 is supported between the tip ends (upper ends) of leaf springs 23a and 23b. Near the upper ends of leaf springs 23a and 23b are provided tracking servo coils 24a and 24b, each of which is wound in the same direction in which leaf springs 23a and 23b extend, that is, in the vertical direction, as shown in FIG. 2. The pick-up device further comprises magnets 25a and 25b which are interposed between respective walls of casing 10 and cylinder 16 at positions opposing coils 24a and 24b.
Accordingly, with the pick-up device 3 shown in FIG. 2 and described above, when an electric current flows through focusing servo coil 20 hollow cylinder 16 moves vertically a distance which is dependent upon the magnitude of the current flowing through focusing servo coil 20. Furthermore, when an electric current flows through tracking servo coils 24a and 24b, leaf springs 23a and 23b move horizontally in either direction a distance which is dependent upon the magnitude of the current flowing through tracking servo coils 24a and 24b.
Thus, with the above-described pick-up device, the light beam emitted from the laser light source 11 through objective lens 13 can be correctly focused on the DAD 1 by supplying the appropriate focusing servo control signal to focusing servo coil 20. Also, if the tracking servo control signal is supplied to tracking servo coils 24a and 24b, the optical axis of objective lens 13 can be adjusted to be coincident with the series of information pits or bumps on the DAD 1.
FIG. 3 is a block diagram showing a modulating circuit for the reproduced signal. In the figure, the signal derived from the output terminal 15 shown in FIG. 2 is supplied to a PLL (phased-locked loop) circuit 31 in which the phase fluctuation component of the reduced signal is detected. The signal from PLL circuit 31 is supplied to a demodulator 32 from which a digital signal is generated. This digital signal generated by demodulator 32 is supplied to a RAM (random access memory) 33 and stored as data therein. Errors in the stored data are corrected by an error correction circuit 34. The error-corrected signal from RAM 33 is supplied to a left-and-right-channel signal separation circuit 35 in which a left-channel signal and a right-channel signal are separated, respectively. The signals separated as set forth above are respectively supplied to D/A (digital-to-analog) conversion circuits 36R and 36L and the analog signals generated by the D/A circuits are respectively supplied to low pass filters 37R and 37L; the filtered analog signals are then respectively supplied to output terminals 38R and 38L. In this manner, digital audio information stored on a digital audio disc, such as the optical type described above, can be reproduced or played back.
A problem with a DAD such as the optical type described above is that the retrieval of information stored on even one of its surfaces requires a very long time period, for example, more than 60 minutes. During such playback, it is not always necessary that a user plays back all recorded information stored on the DAD, the situation may occur where the user wishes to reproduce only a desired portion of the information recorded on the DAD. Thus, it would be advantageous to operate the above-mentioned disc player in a cue (fast forward playback) and a review (rewind playback) mode to that used in tape recorders.